Optimization of Machining Parameters in Machining Inconel 600 by Wire Electric Discharge Machining

Optimization of Machining Parameters in Machining Inconel 600 by Wire Electric Discharge Machining

V. R. Srinivasan, Dr. C. P. S. Prakash Department of Mechanical Engineering, Dayananda Sagar college of Engineering,

Bangalore, India.

Abstract The purpose of this study is to optimize the machining parameters and hence to analyze the characteristics of surface integrity with machining parameters. The specimen material used for this study is Inconel 600. The machining parameters considered for this study are Pulse ON Time, Pulse OFF Time, Peak Current and Bed speed. The Surface characteristics considered for this study are Surface roughness and Recast layer thickness. The experiments are carried out by using Taguchi's L9 orthogonal array. The effect of each machining parameter on the surface integrity is quantified. Based on the effect, optimum machining parameters are identified. A verification experiment is conducted using optimum machining parameters, to show the reduction in surface roughness and reduction in recast layer thickness.

Keywords WEDM, Inconel 600, Recast layer, Design of experiments, Surface characteristic.

1. INTRODUCTION

   Electric discharge machining (EDM) is a thermal material removal process in which the work piece material is removed by local melting or vaporizing small areas at the surface of work piece. The EDM process removes material by controlled erosion of the material by repetitive electric sparks between the work and tool submerged in a dielectric medium. Wire electric discharge machining (WEDM) is a type of EDM process. In WEDM process the tool is a wire that travels through the work piece to cut it.

   Inconel 600 is chosen as work piece material. It is a nickel alloy which is difficult to machine using conventional machining process.

2. LITERATURE REVIEW

   Che-Chung Wang et al [1], explored the feasibility of removing recast layer using etching and mechanical grinding for nickel alloy materials. After experimental machining, SEM was used to measure the recast layer thickness. It was concluded that positive polarity created thicker recast layer then negative polarity and chemical erosion with optimum conditions removed recast layer effectively. It was concluded that EPMA analysis and micro hardness were yielded better results to prove that the recast layer was removed.

   Hsien-Ching Chen et al [2], analyzed the effect of WEDM machining parameters on cutting velocity and surface finish

   of the work piece. Back propagation neural network (BPNN) and simulated annealing algorithm (SAA) was integrated to determine the optimum combination. It was concluded that BPNN was successful in prediction of cutting velocity and roughness. The Pulse ON time was the most influencing factor against cutting velocity and surface finish.

   Zhaoqi Zeng et al [3], experimented combined milling of micro-EDM and micro-ECM. The rough machining was carried out by micro-EDM and the finishing was carried out by micro-ECM. The machining voltage, tool feed rate and machining gap were the machining parameters considered. By micro-ECM finishing the recast layer, micro pores and craters were removed completely. The feed rate had the major effect on surface finish.

   T. Tamura et al [4], analyzed the development of surface modification technology in EDM. The surface defects were eliminated using the sinking electric discharge machine. The machined surface was observed by scanning electron microscope to analyze the recast layer. It was concluded that the recast layer was selectively removed by the dissolution of cobalt as a result of electrolytic action.

   1. Murray et al [5], studied the characterization of microstructural changes, presence of any contaminants and defects at the nano level with the help of transmission electron microscope and laser Raman spectroscopy. It was observed that, during re-solidification of the molten surface, there were some microstructural changes present. The deposition of tool material was detected in the recast layer.

      Farnaz Nourbakhsh et al [6], investigated the effect of machining parameters on process parameters in WEDM. It was concluded that, cutting speed increase with Peak current and pulse interval. Surface roughness increased with increase in Pulse width and decreased with increase in Pulse interval. SEM examination proved that, Zinc coated wire resulted in higher cutting speed and smoother surface finish.

      Based on the Literature survey the main objective of this work is taken as optimization of machining parameters to get smoother surface roughness.

3. EXPERIMENT

   The WEDM process has many machining parameters. Based on the literature survey and the experience of the machine operator it is found that, Pulse ON time, Pulse OFF time, Peak current and Bed speed have the major effect on surface roughness and hence those parameters are focused for the investigation.

   The machining experiments are planned scientifically based on the concept of Design of Experiments (DOE). The type of DOE chosen for this work is Taguchis orthogonal array.

   Number of levels of the orthogonal array is chosen based on the machine capability.

   TABLE 1. INPUT MACHINING PARAMETER VALUES ASSIGNED

   Control Factors	Level 1	Level 2	Level 3
   Pulse ON Time (µs)	3	5	7
   Pulse OFF Time (µs)	22	26	30
   Peak Current (ampere)	1	2	3
   Bed speed (µ/s)	55	70	85

   TABLE 2. L9 ORTHOGONAL ARRAY WITH ASSIGNED VALUES

   Experiment No.	Pulse ON Time (µs)	Pulse OFF Time (µs)	Peak Current (ampere)	Bed speed (µ/s)
   1	3	22	1	55
   2	3	26	2	70
   3	3	30	3	85
   4	5	22	2	85
   5	5	26	3	55
   6	5	30	1	70
   7	7	22	3	70
   8	7	26	1	85
   9	7	30	2	55

   The machining is carried out on Sodic Machine. Molybdenum wire of 0.18mm diameter is used as the tool electrode and De-ionized water is used as the dielectric liquid.

   Inconel 600 is chosen as the work piece material. The work piece dimensions are 10mm x 10mm x 15mm.

   After machining, surface roughness is measured using SURFTEST SJ-210 surface roughness tester. Using Scanning Electron Microscope, images of the machined surface are captured and thickness of the recast layer on each work piece is measured.

   TABLE 4. RESPONSE TABLE FOR S/N RATIO OF SURFACE ROUGHNESS

   Level	Pulse ON Time (µs) (A)	Pulse OFF Time (µs) (B)	Peak current (apere) (C)	Bed speed (µ/sec) (D)
   1	-4.226	-4.816	-5.312	-5.043
   2	-4.885	-4.500	-4.332	-4.805
   3	-5.503	-5.298	-5.160	-4.766
   Delta	1.277	0.798	0.838	0.278
   Rank	1	3	2	4

   Based on the experiments, the optimum set of parameters is A3 B3 C1 D1 to get smoother roughness.

   Level 3 of A, Level 3 of B, Level 1 of C and Level 1 of D are having the smaller signal to noise ratio.

   Thus A3 B3 C1 D1 is the best combination i.e. Pulse-ON Time (7s), Pulse-OFF Time (30 s), Peak current (1 ampere) and Bed speed (55 /s).

   TABLE 5. RECAST LAYER THICKNESS

   Experiment No.	Recast layer thickness ()
   1	2.5
   2	2.4
   3	2.9
   4	3.9
   5	7.0
   6	7.3
   7	9.0
   8	4.6
   9	10.5

   TABLE 6. RESPONSE TABLE FOR S/N RATIO OF RECAST LAYER THICKNESS

   Level	Pulse ON Time (µs) (A)	Pulse OFF Time (µs) (B)	Peak current (ampere) (C)	Bed speed (µ/s) (D)
   1	-8.270	-12.955	-14.164	-15.095
   2	-15.330	-13.924	-13.283	-14.652
   3	-18.925	-15.646	-15.078	-12.779
   Delta	10.665	2.691	1.795	2.316
   Rank	1	2	4	3

4. RESULTS AND DISCUSSION

   Experiment No.	Roughness, Ra (µ)
   1	1.699
   2	1.452
   3	1.745
   4	1.617
   5	1.773
   6	1.885
   7	1.921
   8	1.538
   9	1.895

   Experiment No.	Roughness, Ra (µ)
   1	1.699
   2	1.452
   3	1.745
   4	1.617
   5	1.773
   6	1.885
   7	1.921
   8	1.538
   9	1.895

   TABLE 3. SURFACE ROUGHNESS

   Based on the experiments, the optimum set of parameters is A3 B3 C3 D1 to get minimum Recast layer thickness.

   Level 3 of A, Level 3 of B, Level 3 of C and Level 1 of D are having the smaller signal to noise ratio.

   Thus A3 B3 C3 D1 is the best combination i.e. Pulse-ON Time (7s), Pulse-OFF Time (30 s), Peak current (3 ampere) and Bed speed (55 /s) .

   Pulse ON Time (µs)	Pulse OFF Time (µs)	Peak current (ampere)	Bed speed (/s)	Surface roughness ()
   7	30	1	55	0.812

   Pulse ON Time (µs)	Pulse OFF Time (µs)	Peak current (ampere)	Bed speed (/s)	Surface roughness ()
   7	30	1	55	0.812

   TABLE 7. OPTIMIZED PARAMETERS TO OBTAIN SMOOTHER SURFACE ROUGHNESS

   Pulse ON Time (µs)	Pulse OFF Time (µs)	Peak current (ampere)	Bed speed (/s)	Recast layer thickness ()
   7	30	3	55	2.35

   Pulse ON Time (µs)	Pulse OFF Time (µs)	Peak current (ampere)	Bed speed (/s)	Recast layer thickness ()
   7	30	3	55	2.35

   TABLE 8. OPTIMIZED PARAMETERS TO OBTAIN MINIMUM RECAST LAYER THICKNESS

   The optimum machining parameters obtained from the analysis are verified by conducting verification experiment. The optimum machining parameters results are better than the results obtained from the experimental machining parameters.

5. CONCLUSION

It is concluded that the Pulse ON time has more effect, Peak Current and Pulse-OFF time have lesser effect and Bed speed has least effect on the surface roughness and recast layer thickness. More number of experiments can be performed to analyze the trend of effect of input parameters on the output parameters of the machining process. More number of output parameters can be considered for further analysis.

REFERENCES

1. Che-Chung Wang, Han-Ming Chow, Lieh-Dai Yang, Chun-Te Lu, Recast layer removal after electric discharge machining via Taguchi analysis :A feasibility study, Journal Of Material Processing Technology, 2009, pp 4134-4140.

2. Hsien Ching Chen, Jen Chang Lin, Yung-Kuang Yang, Chih-Hung Tsai, Optimization of wire electric discharge machining for pure tungsten using a neural network integrated simulated annealing approach,Expert Systems With Applications, 2010, pp 7147-7153.

3. Zhaoqi Zeng, Yukui Wang, Zhenlong Wang, Debin Shan, Xiaolong He, A study of Micro EDM and Micro-ECM Combined Milling For 3D Metallic Micro-Structures, Precision Engineering, 2012, pp 500- 509.

4. L. Li, Y.B. Guo, X.T. Wei W. Li, surface integrity characteristics in wire EDM of Inconel 718 at diferent discharge energy, Procedia CIRP, 2013, pp 220-225.

5. J.W. Murray, M.W. Fay, M. Kunieda, A.T. Clare, TEM study on the electric discharge machined surface of single crystal silicon, Journals of Material Processing Technology, 2013, pp 801-809.

6. Farnaz Nourbakhsh, K.P. Rajurkar, A.P. Malshe, Jian Cao, Wire electro-discharge machining of titanium alloy, Procedia CIRP, 2013, pp 13-18.